Method of controlling hybrid vehicle

ABSTRACT

A method of controlling a hybrid vehicle with an engine, a battery, and at least one motor generator. System efficiencies of each of several candidate driving states are calculated. Calculating efficiency when the battery is discharged uses power of the engine P fuel , power drawn from the battery P b,out , and required driving power P demand . Calculating efficiency when the battery is charged uses P fuel , P demand , and power charged to the battery P b,in . P b,out  is calculated using a real battery discharge power P b,out,real , a battery discharge energy efficiency η bd , a historic efficiency η b,pwr  of energy loss when the battery is charged, and a correction coefficient SOC correction  for controlling the battery charge amount. P b,in  is calculated using real battery charge power P b,in,real  battery charge energy efficiency η bc , η bd , and efficiency η in,pwr  when power is consumed in the future. The driving state with the highest efficiency is then selected.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on, and claims priority from, KoreanApplication Serial Number 10-2007-0012034, filed on Feb. 6, 2007, thedisclosure of which is hereby incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The present invention relates to a method of controlling a hybridvehicle.

BACKGROUND OF THE INVENTION

Hybrid vehicles generally use motor generators at slow engine speeds, atwhich the motor generators have better torque characteristics thaninternal combustion engines, and use internal combustion engines atmoderate to fast speeds, at which the engines have better torquecharacteristics. This improves fuel efficiency, as the engine is notused when the vehicle travels at slow speeds.

To control a hybrid vehicle, two system efficiencies are calculated, andthe vehicle is controlled with the higher of the calculated systemefficiencies. The system efficiencies are calculated by Equations 1.

$\begin{matrix}{{\eta_{{sys},{dchg}} = \frac{P_{demand}}{P_{fuel} + {P_{b,{out},{real}}/\eta_{bd}}}}{\eta_{{sys},{chg}} = \frac{P_{demand} + {\left( P_{b,{in},{real}} \right)\left( \eta_{bc} \right)\left( \eta_{bd} \right)}}{P_{fuel}}}} & {{Equations}\mspace{14mu} 1}\end{matrix}$

where:

-   η_(sys,dchg) denotes system efficiency of a driving state of    discharging the battery,-   η_(sys,chg) denotes system efficiency of a driving state of charging    the battery,-   η_(bd) denotes discharge efficiency of the battery,-   η_(bc) denotes charge efficiency of the battery,-   P_(demand) denotes a required driving power,-   P_(fuel) denotes power of the internal combustion engine,-   P_(b,out,real) denotes real discharge power of the battery, and-   P_(b,in,real) denotes real charge power of the battery.

FIG. 2 illustrates a simulation result of a test performed on a hybridvehicle on the basis of the system efficiency calculated according tothe above-described method. The initial state of charge (SOC) of thebattery is 60%. After the simulation ends, the state of charge of thebattery is 53.33%. Relative fuel efficiency is set to 1 as a reference.

This method does not consider energy loss that occurs when the batteryis charged. Therefore, when the SOC of the battery is used in a rangefrom 50 to 70%, system efficiency of discharge is always calculated asbeing higher, and discharging the battery 13 is favored, leading to thebattery being discharged over time.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides a method of controllinga hybrid vehicle. When the battery is discharged, system efficiency ofeach of the candidate points of the driving states is calculated using aratio of how much power P_(fuel) of the engine and power P_(b,out) drawnfrom the battery are used to generate the required driving forceP_(demand). The power P_(b,out) drawn from the battery is calculated bydividing real discharge power P_(b,out,real) of the battery by dischargeenergy efficiency η_(bd) of the battery, history η_(b,pwr) of energyloss when the battery is charged, and a coefficient SOC_(correction) forcontrolling the battery charge amount according to charge capacity ofthe battery.

According to another embodiment of the present invention, when thebattery is charged, system efficiency of each of the candidate points ofthe driving states is calculated using a ratio of how much powerP_(fuel) of the internal combustion engine is used to generate therequired driving force P_(demand) and power P_(b,in) charged to thebattery. The power P_(b,in) charged to the battery is calculated bymultiplying real charge power P_(b,in,real) of the battery by chargeenergy efficiency η_(bc) of the battery, discharge energy efficiencyη_(bd) of the battery, and efficiency η_(m,pwr) when the charged poweris consumed in the future.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of the presentinvention, reference should be made to the following detaileddescription with the accompanying drawings, in which:

FIG. 1 is a schematic view of an exemplary hybrid vehicle;

FIG. 2 is a graph illustrating a result of a simulation on a hybridvehicle on the basis of system efficiency calculated by a traditionalmethod;

FIG. 3 is a graph illustrating a relationship between battery chargecapacity and a correction coefficient; and

FIG. 4 is a graph illustrating a result of a simulation on a hybridvehicle on the basis of system efficiency calculated by an exemplarymethod.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings.

Referring to FIG. 1, a hybrid vehicle includes an engine 1, a firstmotor generator 3, and a second motor generator 5. A planetary gear set7 is provided such that engine 1 is connected to a carrier, first motorgenerator 3 is connected to a sun gear, and second motor generator 5 isconnected to a ring gear. Since first motor generator 3 and second motorgenerator 5 are connected to a battery 13 by inverters 9, and the ringgear of planetary gear set 7 is connected to a differential 11, powercan be taken off by driving wheels.

In a hybrid system, power of the engine I passes through the planetarygear set 7, drives the first motor generator 3, and allows first motorgenerator 3 to generate electricity, such that the power is charged tothe battery 13. Alternatively, power of engine 1 is larger than therequired driving force of the vehicle, and the extra driving forcepasses through planetary gear set 7, drives a second motor generator 5,and allows second motor generator 5 to generate electricity, such thatthe power is charged to battery 13.

Therefore, when battery 13 is charged, energy loss of engine 1, firstmotor generator 3, and second motor generator 5 needs to be considered.System efficiency in consideration of the energy loss when battery 13 ischarged is represented by Equations 2.

$\begin{matrix}{{\eta_{{sys},{dchg}} = \frac{P_{demand}}{P_{fuel} + {P_{b,{out},{real}}/\left( {\eta_{bd} \cdot \eta_{b,{pwr}}} \right)}}}{\eta_{{sys},{chg}} = \frac{P_{demand} + {\left( P_{b,{in},{real}} \right)\left( \eta_{bc} \right)\left( \eta_{bd} \right)\left( \eta_{m,{pwr}} \right)}}{P_{fuel}}}} & {{Equations}\mspace{14mu} 2}\end{matrix}$

where:

-   η_(sys,dchg) denotes system efficiency of a driving state of    discharging the battery,-   η_(sys,chg) denotes system efficiency of a driving state of charging    the battery,-   η_(bd) denotes discharge efficiency of the battery,-   η_(bc) denotes charge efficiency of the battery,-   P_(demand) denotes a required driving power,-   P_(fuel) denotes power of the internal combustion engine,-   P_(b,out,real) denotes real discharge power of the battery, and-   P_(b,in,real) denotes real charge power of the battery,-   η_(b,pwr) will be described below with reference to Equation 3, and-   η_(m,pwr) will be described below with reference to Equation 4.

η_(b,pwr) denotes history of the energy loss when battery 13 is charged.η_(b,pwr) is calculated when a driving state of charging battery 13 isselected, and reflected when efficiency of a driving state ofdischarging battery 13 is calculated. η_(b,pwr) is calculated byEquation 3.

$\begin{matrix}{\eta_{b,{pwr}} = \frac{\int{\left( {{P_{b,{in},{real}}} \cdot \frac{P_{b,{in},{real}}}{P_{fuel} - {P_{demand}/\eta_{c}}}} \right){t}}}{\int{{P_{b,{in},{real}}}{t}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

where η_(e) denotes engine efficiency.

η_(m,pwr) denotes efficiency when the power charged to battery 13 isconsumed in the future. η_(m,pwr) is determined according to history ofefficiency of the first motor generator 3 and second motor generator 5.η_(m,pwr) is calculated by Equation 4.

$\begin{matrix}{\eta_{m,{pwr}} = \frac{\int{\eta_{m}{t}}}{\int{1{t}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

where η_(m) denotes motor efficiency.

On the basis of Equation 2, system efficiency is calculated to adjustthe amount of charging and discharging of battery 13 on the basis of thecharge and discharge capacity of battery 13 by Equations 5.

$\begin{matrix}{{\eta_{{sys},{dchg}} = \frac{P_{demand}}{P_{fuel} + {P_{b,{out},{real}}/\left( {\eta_{bd} \cdot \eta_{b,{pwr}} \cdot {SOC}_{correction}} \right)}}}{\eta_{{sys},{chg}} = \frac{P_{demand} + {\left( P_{b,{in},{real}} \right)\left( \eta_{bc} \right)\left( \eta_{bd} \right)\left( \eta_{m,{pwr}} \right)}}{P_{fuel}}}} & {{Equations}\mspace{14mu} 5}\end{matrix}$

where SOC_(correction) denotes a coefficient that varies according tothe charge capacity of battery 13, as shown in FIG 3.

Speed returning to a desired control center value increases as much asthe charge capacity of battery 13 is distant from the control centervalue. In this way, battery 13 is used such that high energy efficiencycan be obtained and a reduction in the life can be prevented.

FIG. 4 is a view illustrating a simulation result when an FTP72 mode isperformed at an initial SOC=60% according to the method of calculatingthe system efficiency using Equation 5.

That is, when the highest efficiency is selected from the systemefficiencies calculated according to the driving states of the hybridvehicle, and then the hybrid vehicle is driven, since battery 13 is usedin the vicinity of the control center value, it is possible to preventdischarge being favored, and the resulting reduction of the life ofbattery 13. Even though relative fuel efficiency is 0.93, which is lowerthan the value calculated by the prior art Equation 1, the differencebetween the initial SOC and the final SOC of the battery 13 is small.Therefore, considering the difference, better fuel efficiency than thatin the prior art is obtained in terms of the entire hybrid system.

As described above, according to embodiments of the present invention,the energy loss that occurs when the battery is charged and a functionfor adjusting the amount of charging and discharging of the batteryaccording to the charge capacity of the battery are reflected in themethod of calculating the system efficiency, such that fuel efficiencyis improved, the battery is used with high efficiency, and the life ofthe battery is extended.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the presentinvention as defined in the accompanying claims.

1. A method of controlling a hybrid vehicle, the vehicle comprising anengine, a battery, and at least one motor generator, the methodcomprising: calculating system efficiencies of each of a plurality ofcandidate driving states, where one of the driving states comprises thebattery being discharged, where calculating the system efficiency of thedischarging battery driving state uses a power of the engine P_(fuel), apower drawn from the battery P_(b,out), and a required driving powerP_(demand); where P_(b,out) is calculated using a real discharge powerP_(b,out,real) of the battery, a discharge energy efficiency η_(bd) ofthe battery, a historic efficiencyη_(b,pwr of energy loss when the battery is charged, and a correction coefficient SOC)_(correction) for controlling the battery charge amount according tocharge capacity of the battery; and selecting and implementing thedriving state whose system efficiency is highest.
 2. The method as setforth in claim 1, wherein the system efficiency η_(sys,dchg) of thedischarging battery driving state is calculated by:$\eta_{{sys},{dchg}} = {\frac{P_{demand}}{P_{fuel} + {P_{b,{out},{real}}/\left( {\eta_{bd} \cdot \eta_{b,{pwr}} \cdot {SOC}_{correction}} \right)}}.}$3. The method as set forth in claim 1, wherein η_(b,pwr) is calculatedby:${\eta_{b,{pwr}} = \frac{\int{\left( {{P_{b,{in},{real}}} \cdot \frac{P_{b,{in},{real}}}{P_{fuel} - {P_{demand}/\eta_{c}}}} \right){t}}}{\int{{P_{b,{in},{real}}}{t}}}},$where η_(e) denotes an engine efficiency and P_(b,in,real) denotes areal charge power of the battery.
 4. A method of controlling a hybridvehicle, the vehicle comprising an engine, a battery, and at least onemotor generator, the method comprising: calculating system efficienciesof a plurality of candidate driving states, where one of the drivingstates comprises the battery being charged, where calculating the systemefficiency of the charging battery driving state uses a power of theengine P_(fuel), a required driving power P_(demand), and a powercharged to the battery P_(b,in); where P_(b,in) is calculated using areal charge power P_(b,in,real) of the battery, a charge energyefficiency η_(bc), of the battery, a discharge energy efficiency η_(bd)of the battery, and an efficiency η_(m,pwr) when power is consumed inthe future; and selecting and implementing the driving state whosesystem efficiency is highest.
 5. The method as set forth in claim 4,wherein the system efficiency η_(sys,chg) of the charging batterydriving state is calculated by:$\eta_{{sys},{chg}} = {\frac{P_{demand} + {\left( P_{b,{in},{real}} \right)\left( \eta_{bc} \right)\left( \eta_{bd} \right)\left( \eta_{m,{pwr}} \right)}}{P_{fuel}}.}$6. The method as set forth in claim 4, wherein η_(m,pwr) is calculatedby: ${\eta_{m,{pwr}} = \frac{\int{\eta_{m}{t}}}{\int{1{t}}}},$ whereη_(m) denotes a motor efficiency.